1. Field of the Invention
The present invention relates to a fundus blood flow metering method for examining a blood flow state at the fundus of an eye based on the laser Doppler method.
2. Related Background Art
FIG. 9 is a structural diagram of a conventional fundus-blood-flow meter, which is a modification of the slit lamp commonly used for ophthalmic examination. An illumination optical system is placed on an optical path K1 and white light from an illumination-light source 201 is reflected by a perforated mirror 202 to illuminate a blood vessel Ev on the fundus oculi Ea through a contact lens 205 for canceling the refracting power of the cornea of the eye E to be examined, so as to permit observation of the fundus Ea. A measurement-laser-light source 206 for emitting He--Ne laser light is placed on an optical path behind the perforated mirror 202 and measuring light from the measurement-laser-light source 206 passes through the aperture in the center of the perforated mirror 202 to be coaxially aligned with the beam from the illumination light source 201 and irradiate the fundus Ea in a spot shape.
Light scattered and reflected by blood cells flowing in the blood vessel Ev, and by the vessel wall travels through objective lenses 207a, 207b of a light-receiving optical system for stereoscopic observation, which are placed on respective optical paths K2, K3 making an angle .alpha.', and each beam is then reflected by a mirror 208a, 208b and a mirror 209a, 209b to pass through eyepiece 210a, 210b. The light through the eyepieces 210a, 210b is observed as a fundus image by an examiner, and the examiner selects a measured part while observing the fundus Ea through the eyepieces 210a, 210b.
FIG. 10 shows a fundus image to be observed by the examiner. When the blood vessel Ev (to be a measured object) is aligned with a scale SC preliminarily prepared on the focal plane of the eyepieces 210a, 210b within the area illuminated by the illumination light, the axis of the measuring light from the measurement-laser-light source 206 is aligned with the blood vessel Ev, so that the measured part is determined by the spot beam PS from the measurement-laser-light source 206, At this time, reflected light of the measuring light from the fundus Ea is received via optical fibers 211a, 211b by photomultipliers 212a, 212b.
This received signal includes a predetermined beat-signal component resulting from interference between a component undergoing a Doppler shift by the blood flow flowing into the blood vessel Ev and a component reflected by the vessel wall at a standstill, and the velocity of blood flow in the blood vessel Ev is obtained by frequency analysis of this beat signal.
FIG. 11 shows an example of the result of the frequency analysis of received signals measured by the photomultipliers 212a, 212b, in which the axis of the abscissas represent the frequency .DELTA.f and the axis of the ordinates represent the output AS thereof. The relationship among the maximum shift .DELTA.fmax of frequency, the wave vector .kappa.i of the incident light, the wave vector .kappa.S of the received light, and the velocity vector v of the blood flow can be expressed by the following equation. EQU .DELTA.fmax=(.kappa.s-.kappa.i).multidot.v (1)
Therefore, the maximum velocity of the blood flow Vmax can be expressed by the below equation by modifying Eq. (1), using the maximum frequency shifts .DELTA.fmax1, .DELTA.fmax2 computed from the respective received signals by the photomultipliers 212a, 212b, the wavelength .lambda. of the laser light, the refractive index n of the measured part, the angle a between the optical axes of the received light beams K2, K3 in the eye, and the angle .beta. between the velocity vector v of the blood flow and a plane made by the reception optical axes K2, K3 in the eye. EQU Vmax={.lambda./ (n.multidot..alpha.).multidot..vertline..DELTA.fmax1-.DELTA.fmax2.vertline .}/cos .beta. (2)
By carrying out the measurement from two directions in this way, the contribution of the direction of incidence of the measuring light is canceled, so that the velocity of blood flow can be measured at an arbitrary part on the fundus Ea.
For measuring the true blood-flow velocity from the relationship between the intersecting line A between the plane made by the two reception optical paths K2, K3 and the fundus Ea, and the angle P between this intersecting line A and the blood-flow-velocity vector, the intersecting line A needs to be aligned with the velocity vector to satisfy .beta.=0.degree. in Eq. (2). For this reason, an image rotator is placed in the reception optical system to align them with each other on an optical basis.
In practical measurement, it is, however, necessary to check signs of .DELTA.fmax1 and .DELTA.fmax2 computed in this state of .beta.=O. A measurement time necessary for computation of an average flow velocity of an artery blood flow at the fundus Ea is not less than a period of one pulsation in each measurement, which is approximately two seconds for safety. In order to carry out the measurement during this period, the measuring beam must irradiate the measured vessel correctly. For this reason, recent fundus blood flow meters are provided with a tracking means for tracking and capturing the measured part according to eye movement.
In the above conventional example, however, for continuously carrying out the measurement with switching the angle of incidence of the measuring beam to the eyeball, the measurement has to be carried out continuously for the period of time of about four seconds. During execution of this continuous measurement for four seconds, however, there are cases wherein the contrast of the fundus image is temporarily lowered because of nictation, a tear, oil film of the eye, fixation failure of the patient, and so on, so as to lose blood-vessel position information from the fundus video signals during that period, thus making the tracking operation unstable. There is also another problem that the load is great on the patient and the restraint time of the eye of the patient becomes long.
Further, in order to carry out accurate measurement, it is necessary to correct the positional deviation between the measuring beam and the measured blood vessel due to aberration of the eye E to be examined and to set the measurement conditions including adjustment of sensitivity of the photoreceptors for every switching of the angle of incidence of the measuring beam. Therefore, if the measurement is intended to be carried out continuously, the state of the eye E to be examined will be much worse the longer measurement takes. Particularly, in cases wherein the examiner himself or herself needs to adjust the system to effect the correction for the positional deviation between the measuring beam and the measured blood vessel or the like, there will arise such issues as incorrect operation and failure in correct adjustment because of inaccuracy of adjustment procedures.